It is known, from German OS 2,008,142 to join a thread by means of a device, to lift the bobbin off from the main drive roller and keep it separated from the roller and to feed the thread back to the spinning device, the fed back thread being received from the bobbin surface by means of a suction device and being supplied to a pair of draw-off rollers. On a pair of auxiliary rollers, the thread is brought to a specific length whereupon by means of further feedback a predetermined thread length is fed into the open-end spinning device. After the feedback has taken place, the thread is first drawn off from the open-end spinning device by means of the pair of auxiliary rollers, until, after a desired spooling speed has been reached, the thread is introduced into the pair of draw-off rollers.
In this known process, during the joining of a thread only a short period of time is available between the feedback of the thread into the spinning rotor and the start of the thread draw-off from the spinning rotor. When this period of time is exceeded, the thread is over-twisted, so that the thread breaks. On the other hand, in the short period of time for which the thread end can remain in the spinning rotor during the joining phase, twisting cannot continue sufficiently far into the fiber ring so that in the case of a thread draw-off which starts very suddenly, as occurs in the known device, there is likewise the danger of thread breaks.
it is also known to unwind the thread from the bobbin by hand and guide it back into the spinning rotor. Subsequently, the bobbin is re-engaged with its drive roller so that the thread is drawn off from the bobbin (German Offenlegungsschrift 1,560,336) until it is grasped by the traversing thread guide and brought into the region of a catching notch on the pressure roller of the pair of draw-off rollers which brings the thread into the clamping region of this pair of rollers. In this device, the thread is accelerated abruptly both during the start of winding and during grasping by the catching notch and this can lead to thread breaks. Moreover, it is not possible, in a known device, to achieve specific starting of the draw-off, since, in a known state of the art, the bobbin is supported on the drive mechanism and the varying mass of the bobbin causes, during the joining draw-off, a varying slip of the bobbin. Furthermore, tolerances in the shock-absorbers and load-compensating devices for the bobbin arms give rise to different joining ratios at the various spinning units of a machine, and this can make it more difficult to control the thread-joining operation.
An object of the invention is, therefore, to provide a process and a device which avoid these disadvantages and which guarantee specific starting of the thread draw-off during the joining operation even when there is a plurality of spinning units.